#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <ctype.h>

const char * strp_weekdays[] =
{ "sunday", "monday", "tuesday", "wednesday", "thursday", "friday", "saturday" };
const char * strp_monthnames[] =
{ "january", "february", "march", "april", "may", "june", "july", "august", "september", "october", "november", "december" };
bool strp_atoi(const char * & s, int & result, int low, int high, int offset)
{
	bool worked = false;
	char * end;
	unsigned long num = strtoul(s, &end, 10);
	if (num >= (unsigned long)low && num <= (unsigned long)high)
	{
		result = (int)(num + offset);
		s = end;
		worked = true;
	}
	return worked;
}
char * strptime(const char *s, const char *format, struct tm *tm)
{
	bool working = true;
	while (working && *format && *s)
	{
		switch (*format)
		{
		case '%':
		{
			++format;
			switch (*format)
			{
			case 'a':
			case 'A': // weekday name
				tm->tm_wday = -1;
				working = false;
				for (int i = 0; i < 7; ++i)
				{
					size_t len = strlen(strp_weekdays[i]);
					if (!strnicmp(strp_weekdays[i], s, len))
					{
						tm->tm_wday = i;
						s += len;
						working = true;
						break;
					}
					else if (!strnicmp(strp_weekdays[i], s, 3))
					{
						tm->tm_wday = i;
						s += 3;
						working = true;
						break;
					}
				}
				break;
			case 'b':
			case 'B':
			case 'h': // month name
				tm->tm_mon = -1;
				working = false;
				for (int i = 0; i < 12; ++i)
				{
					size_t len = strlen(strp_monthnames[i]);
					if (!strnicmp(strp_monthnames[i], s, len))
					{
						tm->tm_mon = i;
						s += len;
						working = true;
						break;
					}
					else if (!strnicmp(strp_monthnames[i], s, 3))
					{
						tm->tm_mon = i;
						s += 3;
						working = true;
						break;
					}
				}
				break;
			case 'd':
			case 'e': // day of month number
				working = strp_atoi(s, tm->tm_mday, 1, 31, 0);
				break;
			case 'D': // %m/%d/%y
			{
				const char * s_save = s;
				working = strp_atoi(s, tm->tm_mon, 1, 12, -1);
				if (working && *s == '/')
				{
					++s;
					working = strp_atoi(s, tm->tm_mday, 1, 31, 0);
					if (working && *s == '/')
					{
						++s;
						working = strp_atoi(s, tm->tm_year, 0, 99, 0);
						if (working && tm->tm_year < 69)
							tm->tm_year += 100;
					}
				}
				if (!working)
					s = s_save;
			}
			break;
			case 'H': // hour
				working = strp_atoi(s, tm->tm_hour, 0, 23, 0);
				break;
			case 'I': // hour 12-hour clock
				working = strp_atoi(s, tm->tm_hour, 1, 12, 0);
				break;
			case 'j': // day number of year
				working = strp_atoi(s, tm->tm_yday, 1, 366, -1);
				break;
			case 'm': // month number
				working = strp_atoi(s, tm->tm_mon, 1, 12, -1);
				break;
			case 'M': // minute
				working = strp_atoi(s, tm->tm_min, 0, 59, 0);
				break;
			case 'n': // arbitrary whitespace
			case 't':
				while (isspace((int)*s))
					++s;
				break;
			case 'p': // am / pm
				if (!strnicmp(s, "am", 2))
				{ // the hour will be 1 -> 12 maps to 12 am, 1 am .. 11 am, 12 noon 12 pm .. 11 pm
					if (tm->tm_hour == 12) // 12 am == 00 hours
						tm->tm_hour = 0;
				}
				else if (!strnicmp(s, "pm", 2))
				{
					if (tm->tm_hour < 12) // 12 pm == 12 hours
						tm->tm_hour += 12; // 1 pm -> 13 hours, 11 pm -> 23 hours
				}
				else
					working = false;
				break;
			case 'r': // 12 hour clock %I:%M:%S %p
			{
				const char * s_save = s;
				working = strp_atoi(s, tm->tm_hour, 1, 12, 0);
				if (working && *s == ':')
				{
					++s;
					working = strp_atoi(s, tm->tm_min, 0, 59, 0);
					if (working && *s == ':')
					{
						++s;
						working = strp_atoi(s, tm->tm_sec, 0, 60, 0);
						if (working && isspace((int)*s))
						{
							++s;
							while (isspace((int)*s))
								++s;
							if (!strnicmp(s, "am", 2))
							{ // the hour will be 1 -> 12 maps to 12 am, 1 am .. 11 am, 12 noon 12 pm .. 11 pm
								if (tm->tm_hour == 12) // 12 am == 00 hours
									tm->tm_hour = 0;
							}
							else if (!strnicmp(s, "pm", 2))
							{
								if (tm->tm_hour < 12) // 12 pm == 12 hours
									tm->tm_hour += 12; // 1 pm -> 13 hours, 11 pm -> 23 hours
							}
							else
								working = false;
						}
					}
				}
				if (!working)
					s = s_save;
			}
			break;
			case 'R': // %H:%M
			{
				const char * s_save = s;
				working = strp_atoi(s, tm->tm_hour, 0, 23, 0);
				if (working && *s == ':')
				{
					++s;
					working = strp_atoi(s, tm->tm_min, 0, 59, 0);
				}
				if (!working)
					s = s_save;
			}
			break;
			case 'S': // seconds
				working = strp_atoi(s, tm->tm_sec, 0, 60, 0);
				break;
			case 'T': // %H:%M:%S
			{
				const char * s_save = s;
				working = strp_atoi(s, tm->tm_hour, 0, 23, 0);
				if (working && *s == ':')
				{
					++s;
					working = strp_atoi(s, tm->tm_min, 0, 59, 0);
					if (working && *s == ':')
					{
						++s;
						working = strp_atoi(s, tm->tm_sec, 0, 60, 0);
					}
				}
				if (!working)
					s = s_save;
			}
			break;
			case 'w': // weekday number 0->6 sunday->saturday
				working = strp_atoi(s, tm->tm_wday, 0, 6, 0);
				break;
			case 'Y': // year
				working = strp_atoi(s, tm->tm_year, 1900, 65535, -1900);
				break;
			case 'y': // 2-digit year
				working = strp_atoi(s, tm->tm_year, 0, 99, 0);
				if (working && tm->tm_year < 69)
					tm->tm_year += 100;
				break;
			case '%': // escaped
				if (*s != '%')
					working = false;
				++s;
				break;
			default:
				working = false;
			}
		}
		break;
		case ' ':
		case '\t':
		case '\r':
		case '\n':
		case '\f':
		case '\v':
			// zero or more whitespaces:
			while (isspace((int)*s))
				++s;
			break;
		default:
			// match character
			if (*s != *format)
				working = false;
			else
				++s;
			break;
		}
		++format;
	}
	return (working ? (char *)s : 0);
}
